Originally posted by Adam It's specifically better than the F 1.7x teleconverter in this area because rather than having an AF mechanism of its own, the DA 1.4x lets the camera directly control the lens's screwdrive or SDM motor. Thus, unlike with the F teleconverter, you don't have to change the focus setting on the lens to gain access to the full range of focus settings. On top of that, the extra half stop of light allows for faster PDAF operation.
Having an AF mechanism of its own can also be an advantage. I use the F 1.7x AFA quite a bit with AF lenses --
because of the limited range of focus and the speed of locking focus within that range. For me, the AFA works as the most effective focus limiter I've used (much easier to use than the one on the FA* 300/2.8, for ex). I can use AF to make tiny focus adjustments for static subjects (like perching birds) without having to worry that the lens might go into a long lock to lock focus hunt. Even if it does lose fine focus and hunt, the process with the limited focus range only takes a fraction of a second, and I'm back on track.
I even use the AFA with my dedicated macro lenses so I can use AF shooting macro, which is usually thought of as impractical. The focus range with the AFA at macro distances is tiny, and even a lock to lock focus hunt only moves the focus point a few mm, if even that. The added magnification is a bonus.
Also, the AFA does convert f-stop in the camera for exif, and because it does not pass a FL from the lens, it requires that you set FL up when mounted (and holds this FL info from the last time I used the AFA), so FL to match SR is not the problem it is with a 3rd party TC. If you use a lot of different lenses with it, then you must manually change the FL for SR, if you use the same lens, it's essentially automatic.
I guess advantages/disadvantages pretty much depends on one's perspective.
Scott